In Texas, workers' comp insurance companies can do that---doesn't mean it's right or ethical,--- but they do it in some cases. If they initially accepted your case then denied it, I would call the adjuster and ask why they denied it, perhaps through "newly discovered" information,? If you're still in need of medical care to your neck, you would have to fight this through the Division of Workers' Compensation (DWC). As such, call the DWC at (214) 350-9299 or call a local attorney for assistance, such as attorney Kay Goggin at 972-437-1965 or attorney Matt Lewis at 972-644-1111. Good luck.
Contact a TX work comp attorney without delay. In my state (NV) comp insurers can be equitably estopped from raising this issue subsequent to acceptance. Discuss with a TX attorney to ascertain what can be done to fight back.